Permeable membranes may be used in a number of separation or filtration processes when the membranes are properly configured. Properly configured membrane modules may be used for reverse osmosis, ultrafiltration, gas separation, liquid-liquid separation, and supported liquid membrane systems.
Reverse osmosis is a pressure-driven process used for separating solvent from solute and particulate matter. A membrane with a thin dense skin, or barrier layer, over a porous substrate is normally used. Sufficient pressure to overcome the osmotic pressure of the stream being purified or concentrated is used. For sea water (about 3.5% salt), this requires more than 350 psi. and typically about 800 psi. For brackish waters (about 0.5% salt), lower pressure, about 400 psi, is used. The principal uses for reverse osmosis membranes are for the purification of water and the concentration of aqueous solutions. Desalination of sea and brackish water is the major application, accounting for about half the market. Other developing large market opportunities include agricultural water, food processing water, industrial waste streams and municipal waste water.
The driving force for increased utilization of reverse osmosis is its potential for large energy savings compared to other processes. Because a phase change of solvent is not required, as in distillation or crystallization, the process is much more energy efficient. Only one tenth or less of the energy for evaporation is required. However, this advantage decreases for very concentrated solutions which require higher pressure and have lower fluxes through a membrane. For seawater, which has a high salt concentration, the energy saving over the most efficient distillation process is about 50%.
Ultrafiltration is a size-exclusion separation process for both soluble and insoluble matter. Membranes for this process contain pores of a desired diameter, which, depending on size, pass molecules or particles in the range of 10 to 200 .ANG. diameter while retaining larger molecules or particles. Thus, molecules in the range of 300-300,000 molecular weight can be passed or retained while most salts will pass and colloidal particles will be retained. Membranes, or filters, which retain larger species, in the range of 0.02 to 10 mu are generally referred to as microfilters for microfiltration. However, in practice this distinction between ultrafiltration and microfiltration is arbitrary and is not so sharply defined.
Some of the important uses for ultrafiltration include: (1) the recovery of paint from the rinse tank in the electrophoretic painting process--the paint is recycled to the dip tank and the purified rinse water is reused; (2) the breaking of oil-water emulsions common to machinery and metal finishing operations; (3) the recovery of polymer latex wastes and polymeric sizing agents used in fabric finishing; and (4) the separation and concentration of foods, pharmaceuticals and biologicals. The pressures used in ultrafiltration (about 10-100 psi) are lower than in reverse osmosis because there is no significant osmotic pressure generated across the membrane. The hollow-fiber (tube-fed) systems are preferred although limited to about 25 psi in operation. This is the result of the hoop strength limit of the fibers and integrity of the end seals which contain the fibers. Spiral-wound systems can operate at higher pressures (100 psi), but higher flux rates are not achieved because higher pressure drops are necessary to achieve a given flow velocity due to the restricted flow caused by the feed channel spacer screens [R. S. Tutunjian, "Scale-up Considerations for Membrane Processes", Biotechnology, Vol. 3, July 1985, pp. 615-626].
Tubular and plate-and-frame systems have the distinct disadvantages of high hold-up volumes and high space requirements. The concepts of this invention will have the attributes of the hollow-fiber system and higher flux rates will be achieved because higher pressures can be tolerated.
Gas Separation by membranes is a pressure-driven process used to concentrate one or more components in, or from, a multicomponent gas stream. The membranes used for the various processes have a tight dense skin (over a microporous support) which has a higher permeability for one gas over another. Permeability coefficients are a product of the diffusion coefficient and the solubility coefficient. A particular problem in gas separation membranes is that small pores, cracks or fissures may be far more critical than in liquid separations. The low absorption, viscosity and cohesive properties of gases makes the contribution of flow through these channels devastating with many membrane systems. U.S. Pat. No. 4,230,463, "Multicomponent Membranes for Gas Separations" provides for a coating of membrane surfaces with a second material to decrease gas flow through such openings. The advantage in membrane separations as with the other processes is again the large energy savings which result from not having to cause a phase change of the materials in order to effect the separation. Another advantage can be breaking an azeotrope which cannot be broken by simple distillation. A disadvantage may be that the gases produced are not as pure as can be achieved by other means. However, the commercial use of membranes for gas separations has been growing rapidly in recent years. Major uses include: the separation of hydrogen in ammonia plants; separation of carbon dioxide in enhanced oil recovery operations, and in the cleanup of natural gas and landfill gas; and air separation to produce concentrated nitrogen for blanketing of combustible materials and to produce oxygen enriched air for both medical purposes as well as for more efficient combustion and oxidation processes. Both hollow-fiber and spiral-wound systems compete, but the former system has the advantage of about three times the surface area of the spiral-wound units. However, it has been reported that the spiral-wound units can be mounted in any position and that operating personnel can more easily change elements. In contrast, hollow-fiber units are usually installed vertically to prevent the fibers from packing together and are not so readily changed. [G. Parkinson, S. Ushio, and R. Lewald, "Membranes Widen Holes in Gas Separations", Chemical Engineering, Apr. 16, 1984, pp. 14-19.]
Liquid separations (which are reverse osmosis processes), like gas separations, are pressure-driven processes and are based on the selective permeability of one or more components in a multicomponent stream. A process is currently being commercialized by Allied-Signal for the separation of light hydrocarbons from heavy oils. A number of studies have also been reported for the separation of various liquids such as alcohols from water. A variation called pervaporation maintains a gas phase on the outlet side of the membrane by the application of a vacuum to the permeate side. Later the permeate is condensed to a liquid. Most bulk liquid separations have been effected with this process. Examples include the separation of azeotropic mixtures such as the benzene-cyclohexane azeotrope and the separation of very similar compounds such as the isomeric xylenes.
Supported liquid membrane systems contain a liquid impregnated in the micropores of a solid membrane. The liquid is held in place by capillary action. The solid membrane can be a flat sheet, spiral-wound or hollow-fiber type. The liquid selectively permits the passage of a particular component from a mixture. Liquid membranes generally separate by their selective solubility for a given substance. The immobilized liquid generally contains an agent that complexes with the species to be separated, transfers the species through the membrane, then releases the species and returns to repeat the process. For example, Bend Research, Inc. has reportedly developed a process for recovering chromium from electroplating solutions. A waste stream containing 300 ppm chromium was reportedly cleaned to below 10 ppm, and a concentrated stream containing 5 wt % chromium was produced. In addition, Bend Research, Inc. has reportedly developed a process for reducing a uranium leach water containing 100 ppm uranium salt to 10 ppm and producing a 3 wt % concentrate. [Chemical Engineering, Apr. 18, 1983, pp. 9-10.]
Membrane reactor systems are an emerging technology, particularly in biotechnology. As is described in "Membranes In Biotechnology, State of the Art" [A. S. Michaels and S. L. Matson, Desalination, 53, (1985) 231-258] and references cited therein, such systems with appropriate membranes can bring about both biocatalysis and product separation.
A recent assessment of membrane technology and its applications was performed for the United States Department of Energy [S. A. Leeper, D. H. Stevenson, P. Y. -C. Chiu, S. J. Priebe, H. F. Sanchez and P. M. Wikoff, "Membrane Technology and Applications: An Assessment," DOE Contract #AC07-761D01570, NTIS #DE84009000, Feb. 1984]. A number of areas were reviewed in that report, which cited numerous opportunities for significant energy savings through the use of membranes. The applications included waste water treatment and recycle, concentration by reverse osmosis rather than evaporation, recovery of valuable materials from waste wash waters, processing of hot waste streams to allow recycle of water and sensible heat, and concentration of heatsensitive products.
One area cited in the above assessment as having a very large potential for energy savings through the use of reverse osmosis processes is the food industry. In 1976, the food processing industry used a total of 940.times.10.sup.12 Btu. Evaporation and drying accounted for 132.times.10.sup.12 Btu, and it was concluded that 30.times.10.sup.12 Btu could be saved in the food industry by using membranes for two-fold concentration of liquids. It was also noted that "Additional, and perhaps more significant, savings could result from the use of hot process water recycling, reduced energy requirements for transportation, storage and cooling of liquids not usually concentrated (e.g., milk) and reduced costs of vegetable oil extraction by using reverse osmosis recovery of solvent rather than distillation." The reference indicates that among the requirements of the food industry for reverse osmosis applications are low fouling, high temperature capability, and solvent resistance.
The most severe limitation of reverse osmosis systems for the food industry is their fouling problem. Concentration of food products and purification of food process water streams require exposure of the membrane systems to "dirty" streams compared to those normally encountered in cleaner applications such as desalination or water purification, where substantial prefiltration is acceptable.
The only reverse osmosis membrane configurations currently practical for use in this industry are the tubular and plate-and-frame. This is because of their low fouling properties and minimal prefiltration requirements and in spite of their low flux densities and higher costs. The more popular, high flux density and cheaper, spiral-wound and shell-fed hollow-fiber systems are not as applicable to the food industry. The major reason is their difficulty in handling "dirty" separations which have a high propensity to foul the membrane surface.
Other areas reviewed in the referenced assessment were: (1) mining, primary metal recovery, and fabricating metal products; (2) manufacturing textile and leather products; (3) producing pulp and paper products; (4) hydrocarbon extraction and refining; (5) chemical process industries, (6) medical and health care; and (7) domestic, municipal and commercial water treatment.
Prefiltration requirements and fouling of the existing high flux-density membrane systems limit their applicability as well as add significantly to the cost of separations. A primary objective of this invention is the design of a high flux-density membrane separation system with minimal prefiltration requirements and a low propensity to foul. Other advantages will become obvious in the following description.